Staphylococcus aureus, a leading cause of nosocomial morbidity and mortality, has three major resistance mechanisms. The majority of strains are resistant to penicillin and ampicillin; however, in the mid 1970's large numbers of intrinsically methicillin-resistant Staphylococcus aureus (MRSA) were reported. Clinically vancomycin has been the only reliable agent for the treatment of MRSA but its use is associated with high cost, nephrotoxicity, and ototoxicity. More recently strains of Staphylococcus aureus were reported with demonstrated intermediate susceptibility to disk agar diffusion testing with oxacillin (BORSA). However, these strains test as susceptible to oxacillin by the broth dilution method. Preliminary studies performed on 100 clinical isolates of these BORSA strains implicate beta-lactamase activity as the mediating resistance factor. In the presence of a beta-lactamase inhibitor BORSA strains demonstrate susceptibility to semisynthetic pencillins and cephalosporins. Laboratory differentiation of BORSA from MRSA is important to clinicians who must treat the latter with vancomycin while the former may be treated with less toxic agents. This study proposes to examine: (1) the role of antibiotic/beta-lactamase inhibitor combinations in the detection BORSA using agar disk diffusion and microliter broth dilution susceptibility tests on BORSA and control strains to the semisynthetic pencillins and the combination of amoxicillin/clauvulanic acids; (2) the substrate specificity of beta-lactamases from BORSA will be measured by spectrophotometric assay; ultraviolet absorbance spectral will be determined on isolates for oxacillin and other semisynthetic pencillins; and (3) the determination of the isoelectric focusing patterns of BORSA beta-lactamases will be made by gel filtration/polyacrylamide gel electrophoresis; detection of differences in the isoelectric focusing patterns would suggest a qualitative differences in the isoelectric focusing patters would suggest a qualitative difference in the beta-lactamases produced by BORSA compared to other strains of Staphylococcus aureus.